1. Field of the Invention
The invention generally relates to computer audio systems, and more particularly, to a soft switching circuit which gradually connects or disconnects the audio signal to or from another voltage level when a signal is asserted.
2. Description of Related Art
In modern personal computer systems, several devices provide interfaces between the computer system and the human user. Among these devices are systems and equipment designed to allow the user to direct the computer's operations and to provide the user with information regarding the computer's results and status. In particular, many personal computers and applications employ sound to generate warnings or convey information to the user.
Generally, sound is produced by transducers connected to the computer. Amplifier circuitry amplifies audio signals generated by the computer system to provide sufficient current to drive the transducers. To generate and control the audio signals, the computer systems include audio control circuitry, which receives inputs from several sources, including control signals indicating which audio signals should be transmitted to the amplifiers.
Audio systems often include a mute signal for disabling the audio output when audio functions are unnecessary or undesirable. In addition, many audio systems include filter circuits that may be activated and deactivated by the microprocessor to remove unwanted noise. Several methods are available for muting the audio system or activating the filter. Although the complexity of these systems varies, perhaps the simplest and most effective method of muting the audio signal is to simply ground the audio signal before it is provided to the amplifier. Normally, this can be achieved by providing relays between the audio signal outputs and ground. If the relays are deactivated, the audio signal is isolated from ground and drives the amplifier. When the relays are activated by the mute signal, however, the audio signal is short-circuited to ground and the amplifiers are rendered inactive.
Similarly, relays may be used to bypass or activate a stage of a filter. For example, a low pass filter and a band elimination filter may be connected to the audio signal. To bypass the band elimination filter, the node between the filters may be connected to a relay which is also connected to ground. When the relay is deactivated, both filters process the audio signal. When the relay is activated, however, the band elimination filter is bypassed so that only the low pass filter processes the signal.
At the time the audio signal is muted, any signal asserted at the audio input immediately drops to zero. Similarly, when the filter is activated or deactivated, the relay connection to ground causes a sudden variation in the filter circuit impedance, causing a variation in the audio signal voltage. In either case, the sudden change in voltage is amplified, like any other signal, and transmitted to the transducer. The rapid voltage change of the audio signal causes the amplifier to generate an unpleasant pop or snap.
This effect is particularly unpleasant for certain systems that consistently maintain a direct current (DC) offset voltage. Although a DC offset voltage develops at the audio signal outputs, it does not affect the audio signal, as the transducers only react to alternating current (AC) signals and the amplifiers may be AC coupled. If a DC offset voltage is present when the mute signal or the filter signal is activated, however, the connection of the relay to ground causes an immediate change of voltage in the audio system, generating the unpleasant pop from the transducer. As a result, every time the mute signal or the filter signal is asserted, the audio system generates the annoying pop, regardless of whether an audio signal was being transmitted at the time.